
/*
** Copyright (c) 2012 The Khronos Group Inc.
**
** Permission is hereby granted, free of charge, to any person obtaining a
** copy of this software and/or associated documentation files (the
** "Materials"), to deal in the Materials without restriction, including
** without limitation the rights to use, copy, modify, merge, publish,
** distribute, sublicense, and/or sell copies of the Materials, and to
** permit persons to whom the Materials are furnished to do so, subject to
** the following conditions:
**
** The above copyright notice and this permission notice shall be included
** in all copies or substantial portions of the Materials.
**
** THE MATERIALS ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
** EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
** MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
** IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
** CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
** TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
** MATERIALS OR THE USE OR OTHER DEALINGS IN THE MATERIALS.
*/


attribute vec4 gtf_Vertex;
attribute vec4 gtf_Color;
uniform mat4 gtf_ModelViewProjectionMatrix;
varying vec4 color;

void main (void)
{
    const float M_PI = 3.14159265358979323846;
    float x = 2.0 * (gtf_Color.g - 0.5);
    float y = 2.0 * (gtf_Color.b - 0.5);
    float atan_c = 0.0;
    float scale = 1.0;
    float sign = 1.0;
    vec4 result = vec4(0.0, 0.0, 0.0, 1.0);
    const float epsilon = 1.0e-4;

    // Avoid evaluating atan(0, x) for x < epsilon because it's implementation-dependent
    if(x > epsilon || abs(y) > epsilon)
    {
        if(x < 0.0 ^^ y < 0.0)
        {
            sign = -1.0;
        }

        if(abs(y) <= abs(x))
        {
            float c = abs(y / x);

            // Taylors series expansion for atan
            for(int i = 1; i < 12; i += 2)
            {
                atan_c += scale * pow(c, float(i)) / float(i);
                scale *= -1.0;
            }

            result = vec4(sign * atan_c / (2.0 * M_PI) + 0.5, 0.0, 0.0, 1.0);
        }
        else
        {
            float c = abs(x / y);

            // Taylors series expansion for atan
            for(int i = 1; i < 12; i += 2)
            {
                atan_c += scale * pow(c, float(i)) / float(i);
                scale *= -1.0;
            }

            result = vec4(sign * (M_PI / 2.0 - atan_c) / (2.0 * M_PI) + 0.5, 0.0, 0.0, 1.0);
        }

        if(x < 0.0)
            if(y < 0.0) result.r -= 0.5;
            else if(y > 0.0) result.r += 0.5;
    }

    color = result;
    gl_Position = gtf_ModelViewProjectionMatrix * gtf_Vertex;
}

